In current communications networks, several decentralized communications devices, or each of several communications terminals connected with the decentralized communications devices by way of network termination devices, are connected with a parent communications network by way of one or more subscriber connection networks or subscriber access networks, also called ACCESS networks.
In the subscriber access networks, as well as in the parent communications networks, various transmission methods, such as ATM, SDH, PDH, Frame Relay, Ethernet or a combination of these transmission methods, can be used to transmit data.
Subscriber access networks are specifically designed for connecting or allowing subscriber access to parent communications networks, and have interfaces to existing transmission media or special access interfaces designed for high-bit-rate data traffic. Different types of access technologies are used to connect subscribers to subscriber access networks, such as:                connections by way of copper double leads,        connections by way of cable television lines,        connections by way of passive optical networks (PON),        connections by way of active optical networks (AON),        wireless connection networks; i.e., mobile phone systems.        
In the home sector, the copper double lead is often already present. With a corresponding connection technology, these double leads can be utilized for connecting subscribers using high transmission rates, for example, ATM subscribers. Specifically, in this connection, both asymmetrical and symmetrical (digital) transmission methods that have a high bandwidth in the direction of the subscriber are used, ADSL, UDSL, VDSL and SDSL, for example. Such transmission methods are also called xDSL transmission methods. Distribution services, also called “broadband services,” in particular multimedia distribution services, for example radio and television, as well as retrievable services, also called “interactive services,” for example “video on demand” (VOD) or broadband Internet communication, for example video conferencing, can be implemented using such connection systems. The use of xDSL transmission methods to connect subscribers to the subscriber access network is described in the reference as “Xpress-Link—Broadband Access with xDSL Technology,” Siemens AG, 2000, Information and Communication Networks, D-81359 Munich—pages 8 and 9, as an example.
Broadband subscriber access by way of subscriber access networks is described in the reference as “Xpresslink—Broaden your horizons,” Siemens AG, 2000, Information and Communication Networks, Hofmannstrasse 51, D-81359 Munich, Order No. 850001-N8-P60-2-7600. On pages 9 and 10, the structure of a broadband subscriber access network is shown as an example. The subscriber access network shown has several decentralized communication devices, each of which has multiplexer and/or concentrator properties, also called DSLAM, to which one or more subscribers are connected by way of network termination devices, also called NT. Any data traffic that comes from subscribers is brought together, i.e., concentrated, by the respective decentralized communications device and sent to the parent communications network, here an ATM backbone, by way of a uniform interface and, if necessary, by way of a central access device, here an ATM access.
In order to implement the broadest possible spectrum of broadband services and interactive services by way of subscriber access networks, efforts are underway, as part of an initiative called the “FSAN, Full Service Access Network,” to find a generally valid consensus for defining basic requirements on the basis of which broadband subscriber access networks are expected to be introduced over large areas. The definition of the basic requirements is described, for example, in the reference “Full Service Access Network Requirements Specification,” J. A. Quayle, version dated Aug. 18, 1998 (http://www.laps.bt.com/profsoc/access/).
An FSAN network architecture, particularly within the scope of VDSL transmission systems, is described in the following references:                “VDSL System Architecture Specification,” FS0052, revision 0.5 (draft), August 2001;        Architecture Expert Group Contribution, FS0129, revision 2, August 2001.        
The implementation of distribution services by way of subscriber access networks, for example the distribution of multimedia data, e.g., video broadcasts (broadcast TV services) or video on demand (VOD services), is known to persons skilled in the art. A network architecture arranged on the subscriber connection side is shown, as an example, on page 14 of the aforementioned reference “Full Service Access Network Requirements Specification.” The data stream containing both broadband data and narrow-band data linked up to the subscriber by way of the subscriber connection network is first sent to a splitter. The splitter divides the incoming data stream into a broadband data stream, for example a broadband multimedia or video data stream, and a narrow-band data stream, to implement a telephone service. The broadband video data stream is sent to a corresponding broadband network termination device. Several communications terminals, such as set-top boxes, as well as a personal computer, are connected with the broadband network termination device by way of a home network, or Broadband Home Network, installed at the subscriber's location.
Since every set-top box must provide both broadcast TV services and also video-on-demand services, in the case of ATM networks, for example, every set-top box is connected with the broadband termination device by way of two permanently established virtual connections, or Permanent Virtual Connections (PVCs). Each of these connections must be configured for the transmission of multimedia data according to current video standards, e.g., MPEG2, with a transmission rate of 4 to 6 Mbit/s. Thus the broadband network termination device is connected to each decentralized communications device by way of four permanently established virtual connections.
In current ATM communications networks, the individual ATM connections are configured differently depending on the type of service to be implemented in each instance. To implement an interactive service such as video on demand, for example, a permanent virtual connection, VCC (Virtual Channel Connection), is passed from the network termination device NT on the subscriber side by way of the decentralized communications device, hereinafter called a distribution device, and, if necessary, by way of an ATM-Cross-Connect to a VOD server arranged in the parent ATM communications network. The end points of these virtual connections are arranged in the network termination device NT on the subscriber side and in the video server.
In order to implement a broadband service such as a broadcast TV service, for example (which can be installed in addition to an already established interactive service), one or more permanent virtual connections are passed from the broadband network termination device NT on the subscriber side, by way of the subscriber connection line, to the decentralized communications device, i.e., distribution device. In addition, the distribution device is connected, on the network side, with a multiple destination source, i.e., broadcast video source, also called a digital head unit, arranged centrally in the communications network by way of several permanent virtual connections with multimedia data streams (e.g., TV channels) being linked up to the distribution device by way of each of the virtual connections. Using a signaling method implemented between the broadband network termination device, i.e., set-top box and distribution device, for example by means of the standardized IGMP protocol (described in the reference IGMP V2, RFC 2236), the subscriber can select one of the multimedia data streams being linked up in a subscriber-specific manner. This causes the distribution device to send the selected data stream to the broadband network termination device, i.e., to the set-top box in question, by way of one of the permanent virtual connections.
The network architecture on the subscriber side, as described, has the disadvantage that in order to provide interactive services, such as video on demand, a permanent virtual connection for each subscriber has to be established between the broadband network termination device on the subscriber side and the VOD server by way of the distribution device; in other words, a certain amount of transmission resources on the subscriber connection line must be reserved for this service. When the interactive service is not being utilized, the transmission resources reserved on the subscriber connection line between the network termination device and the distribution device are being wasted.